1. Field of the Invention
The present invention relates to an apparatus for and a method of processing a substrate, and more particularly to an apparatus for and a method of processing a substrate, such as a semiconductor wafer or the like, to embed an electric conductor, such as copper, silver, gold, or the like, in fine interconnection recesses defined in a surface of the substrate for thereby forming embedded interconnects on the substrate.
The present invention also relates to an apparatus for and a method of processing a substrate with a processing liquid, such as an electroless plating solution or the like, which is produced and used to form an interconnects-protective layer for protecting surfaces of embedded interconnects that are formed in the manner described above.
Furthermore, the present invention is concerned with a substrate processing apparatus for use as an electroless plating apparatus for forming a protective layer for protecting surfaces of embedded interconnects that are formed in the manner described above.
2. Description of the Related Art
As a process for forming interconnects in a semiconductor device, a so-called “damascene process”, which comprises embedding a metal (electric conductor) into trenches for interconnects and contact holes, is coming into practical use. According to this process, aluminum, or more recently, a metal such as copper or silver, is embedded into trenches for interconnects and contact holes previously formed in an interlayer dielectric film of a semiconductor substrate. Thereafter, an extra metal is removed by chemical mechanical polishing (CMP) so as to flatten a surface of the substrate.
In a case of interconnects formed by such a process, for example copper interconnects formed by using copper as an interconnect material, embedded copper interconnects have exposed surfaces after flattening processing. In order to prevent thermal diffusion of such interconnects (copper), or to prevent oxidation of such interconnects (copper) e.g. in forming thereon an insulating film (oxide film) under an oxidizing atmosphere to produce a semiconductor device having a multi-layer interconnects structure, it is now under study to selectively cover exposed surfaces of interconnects with an interconnects-protective layer (cap material) composed of a Co alloy, a Ni alloy or the like so as to prevent thermal diffusion and oxidation of the interconnects. Such an interconnects-protective layer of a Co alloy, a Ni alloy or the like can be produced e.g. by performing electroless plating.
As shown in FIG. 22, fine recesses (trenches) 4 are formed in an insulating film 2 Of SiO2 or the like which has been deposited on a surface of a substrate W such as a semiconductor wafer. A barrier layer 6 of TaN or the like is formed on the entire surface of the substrate, and then copper plating, for example, is performed onto the surface of the substrate W to fill the fine recesses 4 with copper and deposit a copper film on the insulating film 2. Thereafter, CMP (chemical mechanical polishing) is performed onto the surface of the substrate W so as to flatten a surface of the substrate, thereby forming interconnects 8 composed of a copper film in the insulating film 2. Thereafter, an interconnects-protective layer (cap material) 9 composed of a Ni—B alloy film is formed e.g. by performing electroless plating selectively on surfaces of interconnects (copper film) 8 to protect interconnects 8.
The interconnects-protective layer 9 composed of the Ni—B alloy layer can be formed on surfaces of e.g. copper selectively by using an electroless plating solution which contains nickel ions, a completing agent for nickel ions, and an alkylamine borane or a borohydride compound as a reducing agent for nickel ions, and by immersing a surface of the substrate in the electroless plating solution.
An electroless plating is applied to main filling materials (Cu) for copper interconnects, the formation of a seed layer on a barrier metal, or the reinforcement of a seed (Cu), further the formation of a barrier metal itself, or the formation of an interconnects-protective layer (cap material) for copper interconnects (in any case, Ni—P, Ni—B, Co—P, Ni—W—P, Ni—Co—P, Co—W—P, Co—W—B), or the like. In any electroless plating process, uniformity of the film thickness over an entire surface of the substrate is required.
Generally, electroless plating apparatus of the face-up type employs a small amount of plating solution in one plating cycle, and find it difficult to manage the temperature of the plating solution, in particular, during the plating process. Specifically, the temperature of the plating solution, i.e., the plating temperature, is an important parameter for the processing of substrates, and should preferably be kept at an appropriate level at all times during the plating process. However, as the amount of plating solution used is smaller, the temperature of the plating solution becomes lower during the plating process. The reduction in the temperature of the plating solution greatly affects the plating process, tending to cause variations in the thickness of the plated film. Furthermore, it has been customary for the electroless plating apparatus of the face-up type to operate in one pass, i.e., throw away the plating solution that has been used to plate workpieces in one plating cycle. The one-pass operation, however, is liable to consume a large amount of plating solution, and incur a high running cost.
One solution is to use a circulation process for retrieving and circulating a plating solution for reuse. A retrieving and circulating system, which is based on the circulation process, generally has a circulation tank for storing and circulating a plating solution. The retrieving and circulating system operates by supplying a certain amount of plating solution which has been heated to a given temperature in the circulation tank to a surface, to be plated, of a workpiece thereby to plate a surface of the workpiece, retrieving the plating solution that is left on a plated surface of the workpiece, and returning the retrieved plating solution to the circulation tank.
The electroless plating solution is a mixture of several solutions depending on the electroless plating process, and generally comprises a basic liquid to which a reducing agent, e.g., sodium hypophosphite, is added. For the fabrication of semiconductor devices, it is desirable to use a plating solution to which a sodium-free reducing agent is added for preventing the semiconductor devices from being contaminated by alkali metal. However, a plating solution containing a sodium-free reducing agent is generally unstable and can easily be decomposed, and can more easily be decomposed when kept at a high temperature. When the plating solution is heated to a high temperature in the circulation tank, the plating solution in the circulation tank is easily decomposed, and hence the plating solution stored in the circulation tank, which generally has a large volume, is consumed in a large quantity. If the plating solution, which is retrieved from plated workpieces, is returned in a batch to the circulation tank, then the temperature of the plating solution in the circulation tank is changed, causing variations in the thickness of the plated film.
In the electroless plating process, therefore, the temperature of the plating solution is an important factor for the processing of substrates to be plated, and the amount of plating solution to be used should preferably be held to a minimum required. Once the electroless plating solution, which is a mixture of several solutions depending on the electroless plating process, is produced, the plating solution is highly activated at temperatures higher than a certain temperature, tending to adversely affect the plating process. Consequently, it is desirable to mix a plurality of solutions efficiently into a plating solution immediately before the plating solution is used to plate workpieces.
Rather than the process of heating the plating solution in the circulation tank, an in-line heating process may be employed to heat a necessary amount of plating solution while the plating solution is being supplied, for thereby preventing an increase in the consumption of the plating solution due to its decomposition at high temperatures. However, the in-line heating process generally makes it difficult to manage the temperature of the plating solution, and needs a heating device, such as a considerably large heater or the like, resulting in a large plating facility. If a small heating device such as a heater is employed, then it is necessary to increase the difference between the temperature of the surface of a heat source and the temperature of the plating solution. When the difference between the temperature of the surface of the heat source and the temperature of the plating solution is increased, not only substances are liable to be formed on the surface of the heat source, but also the plating solution tends to be overheated and decomposed locally on the surface of the heat source.